"I don't know of any link directory that has added links out of their own free will."
I had a directory! I put in links for free!
One of the things you do is go to a vertical, look at the top 3 ranking sites, and then create a link to their site for free.
Then when people who want to rank in that vertical do a backlink analysis, they see that the top three sites ALL HAVE LINKS FROM YOUR DIRECTORY! They then think, "I should probably get a link from that directory, too!"
Aside from this devious / intelligent way of giving free links, anybody who wants to have to do a directory has to jump start it
Actually, I had another REAL directory and it was all free links that I was the one who filled in the data. It was a directory of institutions that was more or less related to my niche.
When Panda came out, I removed it because I thought it could be Panda bait.
Looking at it now, I am not so sure that was a good idea. It was fairly comprehensive and useful to users.
The unfortunate thing is that I think that google views hand edited, useful directories as a threat to its business model, and thus has considered them to have negligible importance.